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ABSTRACT Title of dissertation: PERFORMANCE AND LOADS OF VARIABLE TIP SPEED ROTORCRAFT AT HIGH ADVANCE RATIOS Graham M. Bowen-Davies Doctor of Philosophy, 2015 Dissertation directed by: Professor Inderjit Chopra Department of Aerospace Engineering The objective of this work is to develop a refined comprehensive analysis to predict performance and loads of high advance ratio rotors. High speeds, greater than 230 knots, are an important capability of the next generation rotorcraft. To avoid compressibility effects, the rotor tip speed has to be reduced and this means high advance ratios. At increasing advance ratios, the reverse flow region grows wherein the rotor faces flow reversal, high angles of attack and pitch rates, and large variations in dynamic pressures. A large region of flow reversal has implications for trim, performance and loads, which need to be understood in order to design high speed rotorcraft. The focus of this work is to describe the capabilities and limitations of a lifting line-based comprehensive analysis to predict high advance ratio performance with a particular focus on the thrust reversal phenomena. Tip speed can be varied by either radius variation or rotor speed variation. Before focusing on high advance ratios, both approaches are investigated in the context of a standard helicopter configuration in free flight at a representative range of flight conditions. Reducing the rotor radius acts to decrease profile power at the expense of increased induced power. At moderate to high speeds, the decrease in profile power on a reduced radius rotor is greater than the increase in induced power and overall performance can be improved. Reducing rotor speed reduces profile power without impacting induced power and thus larger power reductions can be achieved across the entire flight envelope than with radius reduction. Neither approach allowed trimming the helicopter at advance ratios above 0.4 because of severe retreating side stall, which limited thrust and increased power. Augmented lift and thrust are needed for either concept to achieve significantly higher advance ratios. Vibratory loads were reduced at low speeds, but increased above the baseline at high airspeeds for both variable radius and rotor speed concepts. Variable rotor speed provides larger performance gains and appears more technologically feasible and is the better solution to high advance ratio rotorcraft. The high advance ratio study focused on two wind tunnel tests. The UH- 60A slowed rotor wind tunnel test up to an advance ratio of 1.0 measured rotor performance, airloads and vibratory loads. The analysis is modified and refined by evaluating the correlation to the UH-60A performance and airload test data. The modifications included new aerodynamic models for the fuselage and blade root, yawed flow corrections and refinements to the wake modeling. The refinements are shown to be important for correctly predicting the thrust, drag and power of the UH-60A rotor up to an advance ratio of 1.0. Flap bending loads are sensitive to strong wake interactions on the advancing tip and on the rear of the disk and can only be predicted if the correct root aerodynamic description and location of root trailers are known. The prediction of lift near the blade root is not satisfactory and this is attributed to the highly unsteady airloads in the reverse flow that are not yet adequately understood and require more detailed testing. Mach-scaled high advance ratio tests at the University of Maryland are the second set of test data investigated. The Maryland test included data beyond thrust reversal. The refined analysis is able to predict thrust, including thrust reversal, satisfactorily up to 1.2 advance ratio. Thrust reversal ends when the reverse flow stalls, which is seen in both the test and analysis. The test shows greater post-stall reverse flow lift and this is attributed to lift from dynamic stall. Shaft power and rotor drag are well predicted. The validated analysis is used to study the impact on thrust reversal due to blade twist, shaft angle, root cut-out, reverse flow stall angles, and yawed flow corrections. Negative blade twist shifts the region of thrust reversal to lower collectives while aft shaft angle moves thrust reversal to higher collectives. Increasing the root cut-out delays thrust reversal to higher advance ratios. Yawed flow corrections and increasing the stall limits of the reverse flow airfoil both extend the linear region of thrust reversal. The flap bending moments of the Maryland rotor are dominated by the 4/rev component and this is because the second flap mode is near 4/rev. The 4/rev harmonic content grows with advance ratio and results in large 4/rev vibratory loads at high advance ratio. The blade torsional moment includes higher harmonic oscillations that determine the peak- to-peak torsional loads and these can be simulated in the analysis by simulating dynamic stall in the reverse flow. PERFORMANCE AND LOADS OF VARIABLE TIP SPEED ROTORCRAFT AT HIGH ADVANCE RATIOS by Graham M. Bowen-Davies Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2015 Advisory Committee: Professor Inderjit Chopra, Chair/Advisor Associate Professor James Baeder Professor Roberto Celi Professor Sung Lee Professor Ramani Duraiswami Dr. Anubhav Datta ©Copyright by Graham M. Bowen-Davies 2015 This thesis is dedicated to all those who have endeavored to keep a smile on my face. My parents, my siblings, my wife, my friends and Monty Python. ii Acknowledgments I would be remiss not to take a moment to thank all of those who have played a part in my doctorate degree. None of this would have happened without a professor taking an interest in a Zimbabwean kid from a South African University with a naive desire to study Aerospace. There is no way to adequately thank Dr. Inderjit Chopra for the faith that he has put in me from day one. Always patient, always one step ahead of me, always challenging me to push harder and persistently ignoring me when I told him that there was no way that I could make a deadline. He helped me to solve problems with a firm hand, but without holding mine and continues to provide me, and all his students, the support and time we need. The first time that I met Dr. Chopra he mentioned that he once had a South African student by the name of Andy Bernhard. I am convinced that Andy’s exam- ple is what got me here, and for that I thank him. As well as mentoring my research, Dr. Chopra invited me to take part in the Gamera Human Powered Helicopter (HPH) project. A once-in-a-life-time expe- rience, the project broadened by practical understanding of rotorcraft, challenged my conceptions about what was possible, provided me an outlet to get my hands dirty, found me lifelong friends, taught me more than any class could and delayed my graduation date by about two years. Dr. Chopra and Dean Pines convinced the University to support us and I will be forever grateful. My committee members, Drs. Baeder, Celi, Datta, Lee and Duraiswami, for iii engaging with my research and offering support and for the lessons that I learned under their instruction – thank you. Dr. Nagaraj, VT, has been ever-ready to talk me down from whichever catas- trophic turn my research took that month. He provided his insights and experience in helicopter design to the abstract problems I faced and he was always ready to provide me much appreciated critical feedback. Anubhav Datta, who knew his UMARC analysis code better than anyone and helped me find enough of a foothold to now call it my own. Kumar Ravichandran battled with me against our codes, providing insight and a second perspective. Dr. Robert Ormiston of the Army AFDD always took an interest in my work and pro- vided me valuable feedback and kind words. Dr. Tom Norman of NASA Ames and Hyeonsoo Yeo of ARMDEC were invaluable in providing insight into the analysis of the UH-60A test data. Ben Berry has been my close workmate, my HPH partner in crime, conference buddy and friend. We spent many a late night working together. Joe Schmaus has helped me untangle countless problems, was always at hand to bounce questions off and continues to find the bugs that I hid for him in UMARC. My time at Maryland would not have been as rich without the friends I have made. Brandon Bush showed me around on my first day, took me skiing and tried to teach me drums for Aerospace Has Got Talent. Chen Friedman and Ananth Sridharan pulled me through 14 classes with equal parts laughter and heated debate. Chen – generous and ever ready to help me in a pinch, but never ready to cede the point in many a fruitful argument. Ananth – a frightening ability to out “Math” anyone I know and always happy to help me solve for X once we agree that it even iv matters. Will Staruk, Robbie Vocke, Jared Grauer, J¨urgen Rauleder, Elizabeth Weiner and many others each brought new perspectives and talents that helped my endeavors, and office chitchat that helped less, but enriched my time here. Lizzie Nolan, my wife, has provided me just the right mix of support when I needed to work a little bit harder and longer, and a scolding when I tried working too hard and too long. She was an HPH-widow before we ever married. She has endured all the highs and lows, my long hours and late nights, my frustrations and joys and all of my promises of “one last year”.

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